The invention of this application relates to the stabilization of olefin polymers in the presence of copper.
Olefin polymers have many desirable properties which qualify them for a wide variety of uses. Polypropylene, for example, is a tough, hard, relatively flexible, high-melting polymeric material. It is especially useful as electrical insulation for copper wires and cables. In certain respects, however, the stability of polypropylene is less than satisfactory. Its melt index decreases rapidly and it becomes brittle when kept at elevated temperatures for the time required in milling, calendering, extrusion, injection molding and fiber-forming equipment. This deterioration is particularly serious when the polymers are worked in this fashion in the molten state in an atmosphere of oxygen, i.e., in air. Many well-known antioxidants have been used to inhibit this kind of deterioration, including hindered phenols, secondary aromatic amines, organic phosphites and thiodipropionic acid esters.
The problem is somewhat complicated when the olefin polymer is contaminated by copper or is to be used in contact with copper. When used as insulation for copper wire or cable, for example, polypropylene deteriorates within a few months to the point at which it is virtually useless. As a matter of fact, even in the presence of anitoxidants such as those above, polypropylene degrades rapidly in the presence of copper.
The otherwise excellent mechanical and electical properties of olefin polymers point up the importance of efforts to find ways to prevent this thermal instability in the presence of copper.
U.S. Pat. No. 3,535,257 (Kutner) teaches the stabilization of olefin polymers against this type of deterioration by means of a combination of zinc sulfide, a phenolic antioxidant and a thiodialkanoic acid diester.
U.S. Pat. No. 3,549,572 (Minagawa et al.) teach the use of certain mercapto and thio hydrazides for the same purpose.
An article entitled "Electrical Wire and Cable Plastics Coating -- What's Ahead?" by D. V. Rosato, on page 54 of Wire and Wire Products for March, 1970, teaches the use of ethylene/propylene copolymer instead of polypropylene to ameliorate this problem.
U.S. Pat. No. 3,978,167 (Albright) discloses the use of certain diphosphoramidates of pentaerythritol as flame retardants for synthetic polymers. bis(dialkylamino)pentaerythritol diphosphates are among those disclosed and the bis(diethylamino)pentaerythritol diphosphate is specifically shown. (See Example 3 at column 7.)
The invention herein is an olefin polymer composition comprising a major proportion of an olefin polymer normally susceptible to deterioration at high temperatures in the presence of copper, and a minor proportion sufficient to inhibit such deterioration, of a bis-phosphoramidate having the structure ##STR2## where R is the same or different inert, organic radical and X is oxygen or sulfur.
Such compositions are characterized by unexpected resistance to deterioration at high temperatures in the presence of copper and are, therefore, notably useful as insulation and coating for copper wire.
The olefin polymer may be either a homopolymer or copolymer, but, if the latter, it should comprise at least about 90% of olefin units. Polymers contemplated include polyethylene (low density and high density), polypropylene, polyisobutylene, EPDM polymers, copolymers of ethylene and propylene, copolymers of ethylene and vinyl acetate, copolymers of propylene and vinyl acetate, copolymers of ethylene or propylene with up to 10% of a higher (C.sub.4 -C.sub.6) monoolefin, and terpolymers of ethylene and propylene. The polymers of ethylene and propylene are preferred, and the copolymer of ethylene and propylene is especially preferred.
The bis-phosphoramidates may be prepared by reaction of dichloro pentaerythritol diphosphite with a dialkyl amine, for example, to form the intermediate bis-phosphormaidite. Then, this intermediate is oxidized to the desired bis-phosphoramidate. The following equations are illustrative: ##STR3##
The bis-phosphoramidates herein are those in which R is the same or different alkyl, aryl, cycloalkyl or both R's taken with the nitrogen to which they are attached are one heterocyclic radical. Specific illustrative examples includes methyl, ethyl, propyl, butyl, hexyl, octyl, decyl, tetradecyl, phenyl, tolyl, naphthyl, cyclopentyl, cyclohexyl, methylcyclohexyl, morpholinyl, imidazolyl, piperidyl and the like. Preferred bis-phosphoramidates are prepared according to the above equation when the secondary amine reactant is a dialkyl amine, especially a lower dialkyl amine, i.e., one wherein the alkyl groups are each of 1-6 carbon atoms. The alkyl groups usually are alike although they may be unlike. In most instances, and preferably so, they are each methyl, i.e., the dialkyl amine is dimethyl amine. From about 0.1% to about 1.0% of the bis-phosphoramidate, based on the weight of the polymer composition, should be used, for best results.
Step I represented above may be carried out at a temperature within the range of from about 0.degree. C. to about 120.degree. C. Higher or lower temperatures can be employed, but higher temperatures afford no additional benefit to the reaction and lower temperatures require a longer reaction time. The reaction is exothermic. Generally, a solvent is used, such as benzene or toluene, and the reaction is carried out at the reflux temperature of the solvent.